Fibrous webs consist of more or less irregularly bunched fibers of natural and/or synthetic materials which, apart from their mutual intertwining, have no internal cohesion whatsoever. The strength resulting from the intertwining of the fibers, particularly in low-weight classes, is so poor that the web has practically no inherent stability. Consequently, the web loses its cohesion as soon as it is without external support. The invention relates to a method for bonding such fibrous webs.
DE-OS No. 26 18 245 discloses a process for bonding a fibrous web material. In accordance with the method of this reference, a curable binder is printed in a pattern onto the surface of a fibrous web. The binder is cross-linked by exposing the web to electron irradiation. There are, however, several difficulties associated with the disclosed method which renders it unsuitable for large-scale production. Such difficulties are the result of the high viscosity of available prepolymers which have to be applied onto fibrous webs having poor mechanical stability.
In their cross-linked state, the strength of the available prepolymer binders increases with increasing molecular weight. To obtain a good consolidation of the fibrous web, it is, therefore, desirable to use prepolymer binders having a high molecular weight. However, the viscosity of the prepolymer binder also increases with the increasing molecular weight of the prepolymer. Viscous, high molecular weight prepolymers are difficult to apply to a fibrous web in a uniform distribution in the relatively small amounts employed. This is particularly true when the fibrous web has received no prebonding, and when the prepolymer is to be applied to only certain areas of the web, such as by a printing operation.
In the reference cited above, this difficulty is avoided by employing a prepolymer binder whose viscosity has been reduced by the addition of a solvent. Moreover, in Example 9 of the reference, prior to the addition of the prepolymer binders, the fibrous web is prebonded by the thermal activation of low-melting bonding fibers. Both procedures have disadvantageous aspects.
The use of solvents necessarily involves the high expense of equipment required to keep the air at the work station, and around the production line, free of solvent vapors. There is also the added expense associated with the recovery of excess solvent. Moreover, solvents are flammable and necessarily demand the implementation of elaborate means for the prevention of fires and explosions.
With regard to the thermal prebonding procedure disclosed by the cited reference, the thermal prebonding is irreversible upon the final solidification of the web. Such a procedure results, therefore, in the undesirable reduction or destruction of the bulk of the fibrous web material.
In addition to the technical difficulties discussed above, a large-scale application of the method disclosed by the cited reference is unfeasible since the prepolymer binder after being printed onto the fibrous web, must be cross-linked under a nitrogen atmosphere. Feeds of only 6 m/min are disclosed which are far below the operating speeds currently employed.
As an alternative to solvents it is known to add monomeric low viscosity esters of acrylic acid to the prepolymer binder in order to reduce its viscosity. The viscosity lowering esters are incorporated into the polymer network during the irradiation step. Acrylic acid esters useful for this purpose include butyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, butanediol diacrylate, hexanediol diacrylate, trimethylolpropane triacrylate, and others. Generally, the viscosity-lowering effect of these thinners is the greater, the lower their molecular weight. However, volatility and toxicity also increase as the molecular weight of the ester decreases. Polymers from monomeric acrylic acid esters are usually brittle, fragile products of inadequate mechanical strength. When the acrylic acid ester monomers are polymerized with prepolymer binders, they impair the mechanical properties of the prepolymer binders as well as the flexibility, toughness and softness of the fibrous web material.
Another problem associated with the use of acrylic acid esters results from the fact that residual monomer not reacted during the polymerization of the prepolymer/monomer mixture remains in the polymerization product and can cause skin irritations as well as disagreeable odors.
Due to the kinetics of the polymerization reaction of the monofunctional monomers, unconverted monomer will always be found in the polymerization product. In commercial monomer/prepolymer mixtures, 1% by weight or more of these monomers remains in the polymerization product. With regard to difunctional monomers, the danger of unreacted residual monomer is considerably reduced, but it is still present. Only acrylate monomers which are trifunctional, tetrafunctional, or higher, are incorporated completely into the polymer network. Thus, if possible, the use of monofunctional and difunctional reactive thinners should be avoided. However, mixtures of prepolymers with oligomers or monomers which are trifunctional or higher are always highly viscous, and it has thus far been impossible to process them.